Your father can bequeath to his second wife as property belonging to him. It can not therefore bequeath him 100% of the usufruct .
Indeed, it owns 50% of the property to be acquired with your mother and usufructuary of 50% from your mother because of his death. This usufruct from your mother will automatically shut off to his own death.
Consequently, it can not introduce his wife legatee of the usufruct of the property only up to their part of it, 50% of the property.